The present invention relates to a method for partially depth-wise curing the inner liner of a tire. Additionally, the present invention relates to an improved apparatus for curing the inner liner of a tire.
One common method of manufacturing tires uses a vulcanization bladder. This bladder is inflated with steam or a heated fluid like hot water to press the green tire against the walls of the tire mold. There are several problems associated with the use of bladders in the tire vulcanization process. In particular, the bladder can distort the tire during shaping and affect its position in the mold. This can result in serious quality problems. Another problem with the use of bladders is that they are subject to wear. This results in added costs for replacement of bladders along with the labor and time associated with this replacement. Furthermore, there may be defects in the bladder, such as pinhole leaks. A pinhole leak will allow high temperature steam to escape and contact the inner liner of the tire. This can cause steam penetration through the inner liner into layers between other rubber/reinforced components causing defects. Lubrication materials associated with curing bladder use can also contaminate the tire.
Other disadvantages of using a bladder include a lower heat transfer rate (from the curing medium to the tire) because the wall thickness of the bladder reduces the heat flux resulting in longer cure times, thereby adding to the extra costs of time and energy. The problems and disadvantages of using vulcanization bladders increase when manufacturing larger tires such as tractor tires.
Bladderless molding processes have been used in order to reduce the operating costs associated with a vulcanization bladder and to manufacture larger tires. Bladderless molding methods are disclosed in U.S. Pat. Nos. 4,400,342, 5,127,811 and 5,597,429. While the use of bladderless molding has advantages, there are problems that may arise in this method. Direct contact between the green tire and a fluid under pressure may cause problems due to permeation of the fluid into the unvulcanized tire. The fluid permeation may cause separations between the adjacent layers, or adversely interfere in the adhesion processes between the elastomer material and the reinforcing structures.
A method of curing tires which addresses the problem of fluid permeation is disclosed in U.S. Pat. No. 6,409,959 which discloses a process for curing a tire by providing a primer on the inner surface of the tire and applying heat to the tire to vulcanize the first layer of elastomer and form a layer that prevents permeation of the fluid under pressure into the inside of the tire. Once this impermeable layer is formed, the fluid under pressure is admitted to complete the cure of the green tire.
Another approach to bladderless molding is to create a tire inner liner that is impermeable to these curing fluids. A method of preparing a partially cured layer of rubber in the manufacture of tires is disclosed in U.S. Pat. No. 4,139,405 which teaches the use of radiation to effect a cure or partial cure to the layer of rubber. The cured layer may be an inner liner or the tread layer. The amount or depth of cure is controlled by the intensity of radiation employed. The inner liner may be cured after the green tire is formed with the liner or be cured separately and then used as the inner layer in building up a green tire. Radiation may be used to treat the inner liner of the green tire. By shielding portions of the tire, the areas to be irradiated are controlled. Irradiation however, has no beneficial effect on butyl rubber and certain other rubbers including butyl derivatives.
Another method of preparing a partially cured rubber strip is disclosed in U.S. Pat. No. 4,233,013 which teaches an apparatus for the continuous production of cured rubber strips having a surface design such as for tire treads. The apparatus comprises a means for extruding rubber into a soft, viscous continuous rubber strip, a means for forcing the rubber into molding surfaces under pressure and a means for directing radiation or heat into the viscous rubber while maintaining it under pressure. Essentially, after the rubber is extruded into a continuous strip onto a belt, molding means are applied with pressure onto the rubber and the rubber is exposed to radiation or heat to partially cure the rubber sheet.
Yet another method of partial curing is disclosed in U.S. Pat. No. 5,201,975 which teaches a method of manufacturing a tire with use of a heated pattern ring to mold and partially cure a tread package. The uncured tread package is assembled into a pattern ring to impart a pattern to the tread rubber, the tread rubber is then heated to become partially cured and the tire is further assembled by positioning an uncured carcass within the pattern ring so that the carcass contacts the uncured portion of the tread package.